1. Field of the Invention
The present invention relates to a subcarrier multiplexing (SCM) analog optical transmission system, and more particularly to an SCM analog optical transmission system for optimizing the bias of a laser diode that converts an electric signal into an optical signal.
2. Description of the Related Art
Generally, in a subcarrier multiplexing (SCM) analog optical transmission, a composite second order (CSO) value is changed in accordance with the set bias of a laser diode for converting an electric signal into an optical signal to be transmitted. The CSO value acts as an even-order nonlinear noise component of a second order or higher with respect to a carrier signal and deteriorates a carrier-to-noise (CNR) ratio of the carrier signal. As such, the CSO tend to serve as noise factors in a signal distribution of a wide band, such as a spectrum of a cable television signal. Accordingly, in order to reduce the CSO in the SCM analog optical transmission, it is necessary to optimize the bias of the laser diode.
FIG. 1 is a block diagram illustrating a conventional optical transmission system and includes a modulation section 10, an electric-to-optic converter 12, an adjustment section 26, a optic-to-electric converter 16, a demodulation section 18, a spectrum generation section 24, and an signal generation section 22.
In operation, the modulation section 10 performs a subcarrier multi-modulation of baseband electric signals. At this time, pulse signals generated from the error testing section 22 are inputted to the modulation section 10. The electric-to-optic converter 12 converts the multi-modulated electric signals into optical signals according to the bias of a laser diode and transmits the optical signals through an optical line 14. The adjustment section 26 adjusts manually the bias of the laser diode, which converts the electric signals into the optical signals.
The optic-to-electric converter 16 converts the optical signals transmitted through the optical line 14 into the electric signals. The demodulation section 18 demodulates the converted electric signals corresponding to the modulation operation of the modulation section 10. Meanwhile, the spectrum generation section 24 monitors the carrier-to-noise ratio (CNR) from the electric signals converted by the optic-to-electric converter 16. At this time, the electric signals demodulated by the demodulation section 18 is fed to the signal generation section 22 to test bit error rate. The error testing section 22 in turn forwards the detected error rate to the modulation section 10. To this ends, the signal generation section 22 tests the bit error rate in regard to the demodulated electric signal. Finally, the spectrum generation section 24 displays the electric signals converted by the optic-to-electric converter 16 as spectra.
Note that an operator determines whether to adjust the bias of the laser diode by observing the spectrum of the electric signal displayed on the spectrum generation section 24, then manually adjusts the bias of the laser diode by manipulating the adjustment section 26.
However, in case of adjusting the bias of the laser diode using the conventional optical transmission system as described above, the operator must adjust the bias of the laser diode manually, by directly manipulating the adjustment section 26 and this causes inconvenience. In addition, when the operator directly manipulates the adjustment section 26 while observing the spectrum of the electric signal displayed on the spectrum generation section, it tend to be difficult to optimize the bias of the laser diode precisely.
Accordingly, there is a need for an improved way of optimizing the bias of a laser diode that can be implemented in an optical transmission system.